mqtt: FIX: update hack to handle dummy packet for TCP connect

The zero bytes hack to connect was deprecated by last
commits, so now we send 1 byte initialized to 0 :).

Jira: ZEP-612

Change-Id: I372964bbb102811d33509ad9386d9b360032a180
Signed-off-by: Flavio Santes <flavio.santes@intel.com>